During fabrication of a semiconductor device such as a gallium nitride (GaN) high electron mobility transistor (HEMT), a transition metal such as nickel (Ni) is deposited as a layer of the semiconductor device to realize a Schottky barrier. The Schottky barrier is an energy barrier that provides a rectifying function used to prevent unwanted leakage current from passing through a junction region of the semiconductor device. A thicker noble metal layer for carrying a relatively large current at low resistance is typically deposited over the layer making up the Schottky barrier. However, when a transition metal such as Ni is used as a Schottky contact metal, an intervening layer of a noble metal such as platinum (Pt) or palladium (Pd) is also deposited to form a diffusion barrier layer. Alternately, diffusion layers have been fabricated using an in-situ oxidation process that oxidizes a transition metal during deposition. These diffusion layers and the other layers of metal making up a metal structure for a semiconductor device are generally deposited in a vacuum chamber by thermal evaporation, e-beam evaporation or sputtering. As such, the in-situ oxidation process requires expensive modifications of tools and/or complicated modifications of other processes. Thus, a new process for fabricating a metal structure for a semiconductor device is needed to avoid expensive modifications of tools and/or complicated modifications of other processes.